This invention relates generally to gas insulated equipment and more particularly to vertically-oriented gas insulated transmission lines having means incorporated therein for minimizing the problem of electrical breakdown along the support insulator surfaces.
Compressed gas insulated transmission lines are being used in ever increasing scale in recent years due to the desirability of increasing safety, problems in acquiring right-of-way for overhead lines, and higher power lines required by growing metropolitan areas and growing demands for electrical energy. Compressed gas insulated transmission lines typically comprise a hollow sheath, a conductor within the sheath, a plurality of solid insulating spacers which support the conductor, and a compressed gas such as sulfur hexafluoride or the like in the sheath to insulate the conductor from the sheath. It is also known to provide a particle trap for compressed gas insulated transmission lines as is disclosed in the patent to Trump, U.S. Pat. No. 3,515,939. The particle trap of Trump is used to precipitate out of the insulating gas particles of foreign matter which could adversely affect the breakdown voltage of the dielectric gas.
Although the majority of gas insulated transmission line installations to date have been in a horizontal plane, it is to be expected that, with the increased number of lines being installed, more installations will be made in which the gas insulated transmission line is vertically oriented. This vertical orientation of the transmission line results in problems concerning the presence of conducting particles which problems are not present in horizontally disposed transmission lines. In horizontally disposed transmission lines, low field regions for particle trapping are installed at the bottom of the outer sheath, and gravity and the electric field eventually force these conducting or semiconducting particles into the low field regions where they are trapped or inactivated. In a verticle run of gas insulated transmission lines, particles will, due to gravity, fall down the transmission line and may settle on any insulator surface which blocks the free fall of such particles. The presence of low field regions adjacent the interior surface of the outer sheath would be ineffective to trap and deactivate these particles, as the force of gravity will not preferentially propel particles into such traps. If these particles should lodge on one of the transverse insulating surfaces of the insulators, they may, in turn, lead to initiation of electrical breakdown across the insulator surface followed by a massive short circuit current which will require a costly and time consuming shutdown, repair, or replacement of the equipment. Therefore, it is desirable to provide some means for decreasing the likelihood that the particles which may be present within the outer sheath will lodge on an insulator surface and precipitate electrical breakdown across the insulator.